OK, I've attached a photo of the model taken from the roof. Let me know if this is satisfactory.

Cheers,

Dave P.

Yep. Works fine. Here's what I found.

If the half-span is 43.6" and the full span is 87.2", then the CG at 25% of the MAC is about 1/4" (.2215") aft of where the wing TE intersects the fuselage center line. 20% of the MAC is about 9/16" (.5791") in front of that. Each percent of the MAC works out to be about .1188" or just over 3mm.

Of more concern is that the wing seems to be skewed to the fuselage center line. Did you trammel the wing to the fuse?

According to the measurements I've previously made, the wings are aligned within about 0.15 inches, i.e., the leading edges at the tips (actually at the outer-most wing ribs, which are just a bit inbd of the tip surfaces) are within 0.15 inches of being the same distance aft of an arbitrary fuselage station at BL 0. I tried to line up the fuselage CL perpendicular with the deck boards, but didn't quite get it square. The tip trailing-edge extensions are probably a bit off, as I had to create them from photos; they didn't quite come out the same.

I compared your CG location to the location I used; it's about 1.4 inches fwd of mine, but is very close to the CG location based on the 1/100th-scale Boeing drawing that I had enlarged to use as the basis for my building drawings.

I don't know why there is such a significant difference in the two CG locations, especially when I got reasonably good agreement from the several calculations, which included the lift contribution from the horiz. stabilizer. I don't know how to resolve this conflict except to construct a toss glider and see if I can get any info from that experiment: I'll give this a try.

According to the measurements I've previously made, the wings are aligned within about 0.15 inches, i.e., the leading edges at the tips (actually at the outer-most wing ribs, which are just a bit inbd of the tip surfaces) are within 0.15 inches of being the same distance aft of an arbitrary fuselage station at BL 0. I tried to line up the fuselage CL perpendicular with the deck boards, but didn't quite get it square. The tip trailing-edge extensions are probably a bit off, as I had to create them from photos; they didn't quite come out the same.

I compared your CG location to the location I used; it's about 1.4 inches fwd of mine, but is very close to the CG location based on the 1/100th-scale Boeing drawing that I had enlarged to use as the basis for my building drawings.

I don't know why there is such a significant difference in the two CG locations, especially when I got reasonably good agreement from the several calculations, which included the lift contribution from the horiz. stabilizer. I don't know how to resolve this conflict except to construct a toss glider and see if I can get any info from that experiment: I'll give this a try.

Cheers,

Dave P.

Hi Dave,

You're welcome. It doesn't take me much time to do this as I just load the photo into CAD, then trace the planform and hit the CG command. It finds the centroid (geographic center) which lies on the MAC. From there, it's just a matter of drawing a line between the LE and TE and measuring 25% of that line.

WRT the trammeling, how square is the wing when measured to the center of the tailcone? If it's as close as the nose measurement, then it's probably a slight distortion in the photo and I wouldn't worry about it.

1-1/2" is a pretty significant difference in CG location. Are you saying the my CG is close to Boeing's or yours is?

Keep in mind that the horizontal tail develops negative lift, i.e.: is pushing down to keep the nose up. If you calculated it as having positive lift, then that would explain why your CG location is that much further aft than mine. I never include the tail in my calculations for that reason.

The only time I would include a lifting tail in the calculations is if it has an actual lifting airfoil like is used on an old timer or a model like a Telemaster. I would also include the 'tail' in the calculations when finding the CG for a canard.

A balsa chuck glider should give you a pretty good idea about which CG calculation to use.

Your CG location is very near the location based on the 1/100th-scale Boeing drawing, and corresponds pretty well to the '25% MAC' location given in several of the Boeing (mostly technical-order) sources, though these sources are not all that reliable. Exactly what mass distribution the 'Boeing CG' is based upon, I have no idea, and not being an aerodynamicist, I don't know what the conventional assumptions are.

As far as the alignment of the wings relative to the center plane of the fuselage, I'm satisfied that they are as close to proper alignment as typical model-building tolerances/accuracy can achieve. I didn't measure the distances from, say, the center plane of the vert. stab. to the wing tips, but might do this in the next few days (it takes a bit of doing to install the wings in the fuselage, but it's not a big problem).

The horiz. stab. was built with Boeing BAC 100 airfoils, so it is more/less symmetrical (subject to construction variations), so I don't think it imparts any significant 'negative' lift on the model. In the 'calculators' (from the Web) that I used, I don't know exactly how the horiz. stab. is treated (with respect to lift), but I assume there are some assumptions made, though none of the 'calculators' have any provision for entering an airfoil description. The wake of the air from the wings affects the 'efficiency' of the horiz. stab., so I used a 'low' value in the calculations (there's a place to select an 'efficiency' measure for the horiz. stab. in some of the calculators); choosing 'normal' or 'high' options for the horiz. stab. efficiency doesn't seem to make much of a change in calculated CG locations.

In my 'geometric' determination of the aerod. center and the neutral point from my wing layout drawing, the CG (@25%MAC) turned out to be about 17.2 inches aft of the root LE at BLO, so it's really sort of a dice-roll to figure out where the bloody balance point ought to be.

It sounds like your wing is pretty much aligned, so I wouldn't spend too much more time on that. What I was seeing was probably just an optical illusion caused by the camera's lens.

WRT to the stab and negative lift, keep in mind that in 99.999% of all cases, the wing's airfoil has what's known as a negative pitching moment, meaning that it wants to push the nose of the airplane down.

If the horizontal stabilizer weren't there then the wing would want to flip end over end. If you'd like to see this demonstrated, drop a 36" sheet of balsa from shoulder height and you'll see it flip end or end continuously until it hits the ground. If you put a mark, or piece of tape on the top of one side of the balsa sheet, you'll see that it will always flip in the same direction, i.e.: nose-down.

The job of the horizontal stab is to control this negative pitching moment of the wing by pushing down on the tail. (think of a see-saw type of action) Canards lift on the nose, but essentially perform the same job, i.e.: controlling the negative pitching moment of the wing, which is located at the back of the airplane.

Tailless designs, like the F-106, incorporate reflex in the trailing edge of the airfoil to push down on the tail. This can also be accomplished by adding up elevator trim.

As I say, generally speaking, for models you can ignore the horizontal tail when calculating the CG location because it over complicates the balancing process and usually only leads to more confusion, like what you're experiencing. The exceptions are designs with a lifting horizontal stab like canards, old timers and Telemasters.

I have used the balancing process on all the scratch built models I've built, including WB-57N, B-57B, F-106, Su-27, Su-27IB, Su-35, F-4 and I'll be using it on my F-111. I've also used it to verify the CG for scratch-built models that others have built. In every case this has resulted in a successful test flight, so I have a high level of confidence in this method.

Before the advent of computers, this method was used by cutting out one-half of the wing planform from cardboard, then finding the centroid by balancing it on the end of a pencil. Once found, the MAC was drawn from the LE to the TE, and the desired CG along the MAC was manually measured.

Thanks again for the helpful feedback. I notice that Simons (Model Aircraft Aerodynamics) notes that the most important determinants of the neutral point are the 'mainplane' (wing) and 'tail/foreplane'. Several of the internet/web-based calculators do calculate the tail/stabilizer coefficient, and (I presume) incorporate this measure in the CG location.

In any event, I've chosen a starting CG location about 18 inches aft of the intersection of the leading edges at BL 0. To evaluate this location I've built a 37-inch-wingspan toss glider (photo below), and it seems to balance very close to the corresponding position on the 1/16th model, so will try a few catapult launches with the glider and see how it behaves.

But, as noted above, all this may be a waste of time because the Navy has changed (actually just decided to enforce an existing rule concerning) access to the NAS Whidbey outlying landing field; this site has a hard surface runway of sufficient length to try a flight from.

Under the new enforcement, the only 'pilots' that can fly models from the field (where the local Whidbey Island RC club has flown from for years), are persons with 'base-access' privileges (evidenced by a sticker on their vehicles). This has significantly reduced the number of 'pilots' that can use the facility. Other club members can fly from the field, but must be accompanied at all times by one of the persons with the 'base-access' privileges. I belong to the club, but do not have a base-access sticker (mostly given to retired Navy/military persons who can use other facilities at NAS Whidbey). The RC club officers have stated that the Navy is willing to carry on some discussions about the new enforcement, but no progress is expected very soon. The Navy did not give the club any explanation about why they suddenly decided to enforce this rule (which has been in existence since the club starting flying from the OLF many years ago).

I've done some searching for another hard-surface field, but so far no luck; the closest ones seem to be in Oregon, which poses a problem: I can transport the model OK, but don't have a pilot with an Oregon 'pilot's license'. So at present, the B-47 model is in the 'hangar' (OK, my garage).

Yes, I do know that most CG 'calculators' include the tail planes to find the neutral point and from there the modeler is supposed to find the correct balance point. (CG)

What I've found is that, for the most part, these calculators only breed confusion, such as what you're experiencing right now. Generally speaking, I've found they cause the balance point, or CG, to be aft of where I would locate it.

Naturally, since it's your model and your hard work on the line when it comes to fly yours has to be the final word.

Where the CG is concerned, I always try to live by the old adage that a model that is too nose heavy doesn't fly too well, but a model that is too tail heavy doesn't fly too long.

Having test-flown two scratch-built models that were too tail heavy and having them crash immediately after take-off has caused me to err on the side of caution and if I've got two CG locations I will always choose the one that will cause the model to be too nose heavy. The model may not fly too good, but I know that I'll have a better chance of getting it back down on the ground than if it's too tail heavy.

I like your chuck-glider. It's large enough that you might consider making it RC with ailerons and elevator controls and some means of adding thrust in the form of a single pusher. I think you'll gain a lot of valuable insight into the correct CG as you can test different locations and see how it handles.

I've had two experiences where swept wing CGs determined with a chuck glider were AFT of what was needed for stable flight. I'm convinced the lighter loading has something to do with it. Similar to Dan's comments, I've had the same results with calcs. I haven't seen this topic discussed, but my feeling is that the inner portion of a swept wing, per unit area, is more effective than the outer, thus calculated CGs would be AFT, as they only account for raw area and not the varying efficiency of the wing. There are a few possible reasons for that. First, unless the chord is constant, there are efficiency gains with the larger, inner chord. Second, assuming that the fuse/tail factor into the overall CG location, the wing sweep may not quite have a 1:1 effect of relocating the CG, based on the geometry. Obviously the program has to account for the entire airframe. Again I'm no expert, but I've found CGs determined from calcs and chuck gliders to be considerably AFT, compared with non-swept wings. I've also read a number of posts by builders of swept wing model aircraft, who moved their CG forward from the initial calculated location for stability, after the maiden.

Wow, this is a timely post. I am in the process of converting my swept wing Airhogs chuck glider to dual EDF's and I have had some trouble finding the correct CG point.

Here is a picture of it in pusher form, I had a lot of good flights with it but the jet thing intrigued me. You can see the battery compartment under the front, the hatch is just balsa plywood and Dubro hinges with velcro to close. I may have to move it forward but I hope not.

Since I'm definitely not an aerodynamicist (though an ex-USAF pilot), I don't really understand the complexities of CG location. However, my unschooled idea is that the mass distribution on RC (and other) models must have some reasonably significant effect on the location of the balance point. In the case of my toss/chuck glider, I don't have a way of 'simulating' the mass distribution (motor/fans, batteries, etc.) of my 1/16th model on the glider, so I'm not going to put too much emphasis on the results of my 'tosses.'

In the case of the full-size B-47, CG location/control was extremely important, and was an essential task of the 'back-seat' crew member, who (probably along with the pilot) used a CG-locator 'computer' (kind of similar in appearance to the old E-6B plotters we were taught to use during pilot training) to determine the CG location based on fuel, stores, temperature, field altitude, wind direction/speed, and other conditions during critical flight phases, especially approach and landing.

As noted earlier, I'll let you know any results from the glider 'flight tests', which I hope to perform this weekend.

Since I'm definitely not an aerodynamicist (though an ex-USAF pilot), I don't really understand the complexities of CG location. However, my unschooled idea is that the mass distribution on RC (and other) models must have some reasonably significant effect on the location of the balance point. In the case of my toss/chuck glider, I don't have a way of 'simulating' the mass distribution (motor/fans, batteries, etc.) of my 1/16th model on the glider, so I'm not going to put too much emphasis on the results of my 'tosses.'

In the case of the full-size B-47, CG location/control was extremely important, and was an essential task of the 'back-seat' crew member, who (probably along with the pilot) used a CG-locator 'computer' (kind of similar in appearance to the old E-6B plotters we were taught to use during pilot training) to determine the CG location based on fuel, stores, temperature, field altitude, wind direction/speed, and other conditions during critical flight phases, especially approach and landing.

As noted earlier, I'll let you know any results from the glider 'flight tests', which I hope to perform this weekend.

Cheers,

Dave P.

Good luck Dave I hope it goes well. That is a beautiful model, I wish I could build that well but I'm a hack so I just make ugly planes out of stuff at hand.

On the CG issue I worked a Harris Govt Systems Group back in the late 70's through early 80's. We did several eletronic systems for the B-1B and I remeber the engineers from Boeing and the USAF discussing CG and equipment encroachment into the empenage of the airplane. They were very serious about this and the balance of the airplane with and without payload.

I wish I still knew some of those guys, they could calculate the answer in short order.

Hi Ed! I used the 2W20 motors; they seem to produce enough thrust. Happy New Year!! Dave P.

Cool, your B-47's thrust is around 30.056 pound @ full throttle calculated by using 1.623 pound of model for each 1.0 pound of thrust I used for recip d/f. Since the recip pulse cycle is not present the effeciency is greater than 1.623 but I grace that as the smaller fans are effecient in a narrow bandwidth.

Wish we could get our AUW down that low...we are 48# to 52# at 1:8.7669. Since we have held off on batteries their weight has been reduced by at least 25% with a capacity increase of no less than 30%.

Hi Ed! Wow! I can understand why you're going to use the RATO...
Will sure be interested to get more photos of the 'build', and your video of the first flight!! Happy
New Year, Dave P.

Dave,

RATO is for visual effect only. Its not legal to use anything for thrust. Those are smoke generators,
not rockets. When we get a still day I will shoot a video showing the smoke trials as I cut them down
to find proper burn time for use during a take-off. Manufacturer said they develop full effect within
15 seconds after ignition. The 1:1 used as many as 33 RATO in the horse collar attachment.

Have you fast taxied yet? It would be neat to see you go to Whidby's old seaplane tarmac with it. Did you ever ask anyone in
the club to make arrangements? It would be fabulous to see you doing fast taxies.

Did you install brakes? I've integrated Kavan Electric brakes on both rear mains. It fits perfectly within the Williams Bros. wheel
case. Bought a baby ESC for proportional braking though after reading recent user reports where in they simply switched on or
off with a servo without experiencing any issue. As long as they don't lock up with our 42#~43# they will work perfectly.

Recently you were most concerned about your CG to the extent it had you grounded until you felt more comfortable with
data from Boeing. Well, I finally found the CG declaration on Boeing's frame number @623.7.

Thanks for the input! With the use of a couple of the on-line CG calculators, I was able to choose what I think is a good starting location for the GC. I physically balanced the model at that location (which is very close to the fuselage station shown in the drawing). However, opportunity to fly the model remains nil, as the Whidbey Island RC club's field is more/less unavailable to us members who do not have access to the NAS Whidbey base. I'll post any updates if anything changes in the future.